The objectives of the proposed studies are to define the pathogenesis and immune response to Toxoplasma infection acquired by ingestion and to determine whether the immune response protects the host and prevents transmission to the fetus in utero when Toxoplasma is subsequently ingested. Infection with Toxoplasma gondii acquired by the gastrointestinal route will be utilized as a model since the major natural routes of acquisition of infection are ingestion and transplacental transmission. This study will utilize a murine model and include assays for the presence of intestinal secretory IgA and serum IgA specific for Toxoplasma, transformation of lymphocytes from mesenteric lymph nodes and spleen to Toxoplasma antigen, homing of lymphocytes to the gastrointestinal tract, and microbicidal capacity of mononuclear phagocytes (peritoneal macrophages and Kupffer cells). To determine whether the immune response prevents transmission of infection with subsequent ingested challenge, nonpregnant and pregnant mice will be infected and challenged with intestinally administered cysts and fetal tissue examined for infection. If the immune response to ingested T. gondii prevents transmission when cysts are subsequently administered orally, immunization procedures which stimulate the immune response will be developed and employed to prevent infection. To establish the relevance of these studies to prevention of infection in humans, components of the immune response to infection in humans will be studied. These studies will include assays for presence of specific anti-Toxoplasma secretory and serum IgA, and microbicidal capacity of blood monocytes and Kupffer cells.